Method of fabricating and soldering stainless steel parts

ABSTRACT

The two faces of a sheet or thin strip of stainless steel are covered with a thin coating of lead, and is used for constituting the tubes, collector, water-boxes as well as the pipes of a radiator, and the parts thus made are soldered together with a tin-lead alloy by the same operational cycle as with corresponding parts made of brass or copper.

United States Patent Chartet I Sept. 12, 1972 [54] METHOD OF FABRICATING AND SOLDERING STAINLESS STEEL PARTS [72] Inventor: Andre Chartet, Meudon, France [73] Assignee: Societe Anonyme Des Usines Chausson, Asnieres, France [22] Filed: June 23, 1970 [21] Appl. No.: 49,154

[30] Foreign Application Priority Data June 30, 1969 France ..6922044 [52] US. Cl ..29/157.3 R, 29/482, 29/502, 29/527.2, 29/DIG. 37

[51] int. Cl. ..B21d 53/02 [58] Field of Search ..29/157,3 R, 504, 502, 527.2, 29/DIG. 37, 482

[56] References Cited UNITED STATES PATENTS 3,128,546 4/1964 Schuster et al. ..29/504 X 2,498,827 2/1950 Shaw et al ..29/l57.3 R 2,937,438 5/l960 Lemon ..29/502 X 3,242,565 3/1966 North et al ..29/502 X 3,465,424 9/ l 969 Deringer .29/527.2

Primary Examiner-John F. Campbell Assistant ExaminerDonald C. Reiley, III Att0rneylmirie and Smiley [57] ABSTRACT 6 Claims, N0 Drawings METHOD OF FABRICATING AND SOLDERING STAINLESS STEEL PARTS BACKGROUND OF THE INVENTION It is well known that the methods for manufacturing products such as radiators are based upon the utilization of copper and copper alloys, principally red copper and brass, since those metals and alloys, are relatively corrosion resistant and moreover easy to shape for constituting tubes, collectors, water-boxes, pipes, etc. which are easy to secure together by soldering with alloys of lead and tin.

Copper and copper alloys are costly and attempts have been made to use more common metals, namely ferrous metals, but this introduces a very difficult metal corrosion problem. Therefore, although ordinary ferrous metals can be easily assembled by means of soldering, they have not been used except, at the most, for manufacturing cooling fins, i.e., indirect exchange surfaces which are never in contact with the liquid circulating in the heat exchanger.

Even for this restricted use of the manufacture of only the cooling fins, it is necessary to coat the outside of the core to protect said fins against rust. This coating is applied when the radiator is used in a motor vehicle even though the radiator is exposed to the protection of oil vapors, since in nearly all cases it is placed in the motor compartment of the vehicle.

Stainless ferrous alloys, namely stainless steels, have been known for a long time but up to now such steels could not practically be used in series made radiators even if their stainless state made them a priori usable for making collectors, water-boxes, and tubes for radiator cores, because soldering of stainless steels has been found very difficult. Actually, it is possible under certain conditions to connect two parts of stainless steel by soldering, but it is necessary previously to submit the parts to a very powerful pickling and fluxing step, and then to proceed immediately after with said soldering.

It has been found that in the manufacture of radiators, such a process could not be used since all of the soldering of a radiator can not simultaneously be made. Actually, if soldering of the ends of the tubes in the collectors could otherwise be effected, it would be difficult for the above-mentioned reasons and would induce, by using a stainless steel collector, the passivation of the non-soldered parts of the collectors on the ends of the tubes. Therefore, it would be necessary to submit the assembled cores once again to new pickling and fluxing operations in order to enable the soldering of a water-box on a collector made of stainless steel.

It is apparent that these very difficult and frequent operations result in a very high manufacturing cost. Moreover, it is difficult to stamp stainless steel and special tools are required for doing this work.

SUMMARY OF THE INVENTION The present invention is directed to a new method for manufacturing heat exchangers and, more particularly, cooling or heating radiators for vehicles, which has the great advantage of enabling the use of stainless steel for making the entire radiator or parts thereof so that the radiator may still, if necessary, comprise parts of cupreous metals. Furthermore, existing manufacturing equipment, such as the shaping tools and the assembling machines, may be used and with the certainty that the radiators thus produced will retain not only the same qualities, but also'improved qualities with respect to those made of brass and copper. This result is obtained while greatly reducing the manufacturing cost.

According to the invention, for making at least some of the parts of a radiator, the method comprises the steps of taking a sheet or strip of stainless steel as the basic material, treating said sheet or strip to cover the faces thereof with a coating of non-ferrous metal or lead alloy and adapted to be wet by soldering alloys containing tin and lead, shaping said parts from the sheet or strip of coated stainless steel and assembling the parts of the radiator through a conventional soldering process.

DETAILED DESCRIPTION OF THE INVENTION For the manufacture, according to the invention, of various parts of cooling and heating radiators for motor vehicles, for example water-boxes and connecting pipes, collectors on which said water-boxes have to be fixed, and, also, circulation tubes, 21 stainless steel is used which contains no nickel and a proportion of chromium of at least 13 percent; for example, a steel known under the name Z 8 C 17 of AFNOR classification may be used. The steel is submitted to a treatment in order to make possible the fitting together of the different parts of the radiator by means of solderings, namely tin-lead solderings.

A satisfactory procedure is to submit the stainless steel to an initial treatment comprising degreasing in an alkaline or acid bath, the composition of which may be variable. Baths used for degreasing ordinary iron sheets are perfectly satisfactory. Stainless steel is then submitted to a chemical pickling in a fluonitric aqueous bath which may contain from 2 to 10 percent of hydrofluoric acid and from 5 to 30 percent of nitric acid, this pickling being performed at ambient temperature. The sheets or strips of stainless steel thus depassivated may be stored in a dry room for a very long time.

During the proper treatment, the sheets or strips of stainless steel are submitted to a second pickling operation in a bath of the same kind asthe bath of the initial treatment, or preferably this second pickling is made by dipping in an aqueous bath containing 100 to g/liter of hydrochloric acid, i.e., a tri or quadrimolar bath. Following the second pickling, the stainless steel is rinsed with fresh water, then immediately submitted to a fluxing operation by introduction into a molten salt bath containing 85 to parts by weight of zinc chloride and 15 to 25 parts by weight of ammonium chloride.

As soon as the steel leaves the fluxing salt bath, it is passed or dipped into a molten bath containing from 8 to 40 parts per weight of tin and from 60 to 92 parts per weight of lead. At the output of the tin-lead bath, the sheets or strips are submitted to a powerful wiping action while the tin-lead alloy is still molten. This wiping action may be carried out by blowing hot air or by means of rolls or by a combination of these two opera tions. The thickness of the alloy with a high percentage of lead which is thus deposited must be as thin as possible because it is only desired to form on the stainless steel surface a dependent coat suitable for subsequent wetting by soft solder. As the lower limit, a molecular coat is sufficient but in practice for convenience in manufacturing, this coat may be between about 0.5g. and in thickness.

The constitutive parts of the radiator are shaped by rolling with regard to the ducts, or by stamping with regard to collectors and water-boxes, only after the basic metal has been coated on the two faces thereof. Actually, and surprisingly, it has been found that it is much easier to coat strips or continuous sheets of stainless steel with an added metal when said strips or sheets are not previously mechanically worked. Moreover, the stamping of stainless coated steel is made considerably easier by said coating alloy, the stamping operations particularly, can be made in a manner as simple and with the same tooling used for working non-ferrous alloys such as brass. This is generally not the case when parts are stamped from a non-coated stainless steel sheet.

The parts of coated stainless steel, made according to the invention, may be fitted together, or with parts of other metals such as copper or brass, with exactly the same procedure as for assembling said parts of non-ferrous metals. Therefore, the parts are submitted to fluxing, then they are joined with a soft solder comprising a tin-lead alloy, the dependent coat covering both faces of the element of stainless steel thus ensuring the intimate connection with the soft solder.

The invention thus enables the manufacture of radiators of which all the elements may be made of stainless steel, thereby providing radiators having improved mechanical characteristics and making possible operation under higher pressure conditions for the water circuits. Moreover, this result is obtained at a lesser cost than for analogous radiators made of non-ferrous metals, for example brass.

I claim:

1. A method of making products of which at least some parts are made from sheets of stainless steel, which method includes the steps of providing a sheet of stainless steel containing no nickel and at least 13 percent by weight of chromium,

deoxidizing the sheet of stainless steel, and immediately covering said deoxidized sheet on both faces thereof with a lead alloy in molten form, whereby the covered sheet is wettable by tin and lead solder,

forming parts of said product from the covered sheet,

assembling together the formed parts to constitute said product, and then tin and lead soldering the assembled parts.

2. Method according to claim 1, wherein the sheets are covered with a thincoat of a lead-tin alloy.

3. Method according to claim 1, comprising the further step of submitting the sheet, before deoxidizing, to degreasing, then to pickling in a fluonitric bath whereby the steel is depassivated.

4. Method according to claim 3, wherein the step of deoxidizing the sheets includes subjecting the depassivated sheets to a pickling bath containing from three to four moles of acid by liter, then to a fluxing step in a molten bath containing zinc and ammonium chlorides.

5. Method according to claim 4, wherein the step of covering includes dipping the fluxed sheet in a molten bath of a high lead percentage alloy, and wiping the sheet whereby the coating has a thickness between 0.541. and 201.1

. A method of making products comprising at least some parts made from sheets of stainless steel and some portions of copper alloys, said stainless steel containing no nickel and at least 13 percent by weight of chromiurn, which method includes the steps of pickling the sheets of stainless steel in a fluonitric bath; keeping the pickled sheet in a dry atmosphere before use; at the moment of use;

pickling the dried sheets in an aqueous bath with at a least one acid selected from the group consisting of fluonitric and hydrochloric acids;

rinsing the pickled sheets with water; and immediately after said rinsing, the further step of dipping the rinsed sheets in a bath containing a mixture of molten zinc and ammonium chlorides and then removing the sheets coated with the mixture of molten zinc and ammonium chlorides;

dipping the coated sheets in a bath containing a molten lead and tin mixture and then removing the sheets covered with the molten lead and tin mixture;

wiping the covered sheets of the excess of still molten tin and lead;

forming the parts from the wiped sheets and assembling the same with the portions of copper alloys into an assembled product;

submitting the assembled product to a soft fluxing with a flux of the type used in tin-lead soldering of copper members; and

heating the same to the soldering temperature. 

2. Method according to claim 1, wherein the sheets are covered with a thin coat of a lead-tin alloy.
 3. Method according to claim 1, comprising the further step of submitting the sheet, before deoxidizing, to degreasing, then to pickling in a fluonitric bath whereby the steel is depassivated.
 4. Method according to claim 3, wherein the step of deoxidizing the sheets includes subjecting the depassivated sheets to a pickling bath containing from three to four moles of acid by liter, then to a fluxing step in a molten bath containing zinc and ammonium chlorides.
 5. Method according to claim 4, wherein the step of covering includes dipping the fluxed sheet in a molten bath of a high lead percentage alloy, and wiping the sheet whereby the coating has a thickness between 0.5 Mu and 20 Mu .
 6. A method of making products comprising at least some parts made from sheets of stainless steel and some portions of copper alloys, said stainless steel containing no nickel and at least 13 percent by weight of chromium, which method includes the steps of pickling the sheets of stainless steel in a fluonitric bath; keeping the pickled sheet in a dry atmosphere before use; at the moment of use; pickling the dried sheets in an aqueous bath with at least one acid selected from the group consisting of fluonitric and hydrochloric acids; rinsing the pickled sheets with water; and immediately after said rinsing, the further step of dipping the rinsed sheets in a bath containing a mixture of molten zinc and ammonium chlorides and then removing the sheets coated with the mixture of molten zinc and ammonium chlorides; dipping the coated sheets in a bath containing a molten lead and tin mixture and then removing the sheets covered with the molten lead and tin miXture; wiping the covered sheets of the excess of still molten tin and lead; forming the parts from the wiped sheets and assembling the same with the portions of copper alloys into an assembled product; submitting the assembled product to a soft fluxing with a flux of the type used in tin-lead soldering of copper members; and heating the same to the soldering temperature. 